A spring fitted to a nut functions to press the nut against an inner surface of the two mutually opposing rail flanges that define a slot in the rail, so that as a result of contact between the nut and the flanges, the nut will remain in a position to which it is set along the rail. A screw can be inserted through the slot and screwed into a nut exposed in the slot.
In the simplest case, the spring-biassed nut is fitted by inserting the nut from one end of the rail. It is desirable, however, to be able to fit the nut generally radially into the rail through the slot at any chosen position therealong. The nut shall be held against rotation in the rail and in order to be on the safe side, it is necessary to avoid nut constructions which include movable parts by means of which the nut is fitted to the rail.